The present invention relates to an improved solenoid. Conventional solenoids have an electrically energizable coil consisting of one or more layers of windings about a non-magnetic form producing an electromagnet when the coil is energized. A magnetic armature extends from this form and moves within it. In one construction of solenoid this armature moves to a retracted position when the coil is energized and to an extended position under urging from a biasing means when the coil is de-energized. The distance between these positions is known as the "throw" of the solenoid.
Such solenoid device is widely used for operating circuit breakers, track switches, valves and many other electro-mechanical devices. They may be used to operate door locks on cars, in automatic nailer and stapler machines, as electrical motor starters, as contactors on starting equipment, and on fuel racks for diesel engines and the like.
Problems with such conventional solenoids however have limited their applications and usage. For example, with the armature in extended position, because the inner end of the armature is at its position most remote from the corresponding magnetic pole, there is relatively little starting power available to attract the armature into the coil when the coil is energized. As well, the throw is limited by the amount of magnetic strength available when the coil is energized. In practical terms the throw may for instance only be about an inch. In order to overcome these limitations, the current to windings may be increased. Increased current however results in increased heat being generated when the coil is energized, which heat reduces the ability of the coil to function as an electromagnet and reduces the life of the device and components which may surround it.
In our invention as described in our co-pending U.S. patent application Ser. No. 07/890,820 filed Jun. 1, 1992, an improved solenoid construction was described and illustrated which overcame these problems. In particular, the coil end of the armature has secured to it a pre-determined length of rigid non-magnetizable material. A second, electrically energizable coil is energizable in a manner opposite to that of the first coil, and is secured to the other end of the length of non-magnetizable material so as to move with the armature. When this second coil is energized, it moves from a position adjacent the first coil (with the armature in extended position), to one spaced therefrom with the armature in retracted position. The energization of the two coils provides magnetic repulsion between adjacent, like magnetic poles of the two coils to produce excellent starting power for the armature. This improved solenoid construction also permits an increased throw compared to conventional solenoids.
One problem with such a construction of solenoid, for particular applications which require a solenoid in which the armature is very strongly held in retracted (coil-activated) position is that the armature is not locked in this position as strongly as would be the case with a conventional solenoid. This is because, when the armature is in this retracted position with the coils energized, the locking of the (heavier) armature in this retracted position is still achieved mainly through the effects on the inner end of the armature by the magnetic field created by the first coil.
It is an object of the present invention to provide an improved construction of such a solenoid which will have significantly increased power in holding the armature firmly in retracted, locked position.